1. Field of the Invention
This invention relates to a process for preparing a catalyst which comprises combining catalyst components or precursors thereof in an aqueous medium to form a catalyst precursor mixture, feeding the mixture to a shaping apparatus, and shaping the mixture to form particles. The invention furthermore relates to catalysts obtainable by this process.
2. Prior Art
A common challenge in the design and production of heterogeneous catalysts is to find a good compromise between the effectiveness and/or accessibility of the active sites and the effectiveness of the immobilising matrix in giving the catalyst particles sufficient physical strength, i.e. attrition resistance.
The preparation of attrition resistant catalysts is disclosed in several prior art documents.
GB 1 315 553 discloses the preparation of an attrition resistant hydrocarbon conversion catalyst comprising a zeolite, a clay, and an alumina binder. The catalyst is prepared by first dry mixing the zeolite and the clay, followed by adding an alumina sol. The resulting mixture is then mixed to a plastic consistency, which requires about 20 minutes of mixing time. In order to form shaped particles, the plastic consistency is either pelletized or extruded, or it is mixed with water and subsequently spray-dried.
The alumina sol disclosed in this British patent specification comprises aluminium hydroxide and aluminium trichloride in a molar ratio of 4.5 to 7.0. This type of alumina sol will be referred to in the present specification as aluminium chlorohydrol.
U.S. Pat. No. 4,458,023 relates to a similar preparation procedure, which is followed by calcination of the spray-dried particles. During calcination, the aluminium chlorohydrol component is converted into an alumina binder.
WO 96/09890 discloses a process for the preparation of attrition resistant fluid catalytic cracking catalysts. This process involves the mixing of an aluminium sulphate/silica sol, a clay slurry, a zeolite slurry, and an alumina slurry, followed by spray-drying. During this process, an acid- or alkaline-stable surfactant is added to the silica sol, the clay slurry, the zeolite slurry, the alumina slurry and/or the spray-drying slurry to prevent gelling or polymerisation of the slurry ingredients before spray-drying.
CN 1247885 also relates to the preparation of a spray-dried cracking catalyst. This preparation uses an aluminous sol, a molecular sieve slurry, clay, and an inorganic acid. In this process the aluminous sol is added before the clay and the inorganic acid, and the molecular sieve slurry is added after the inorganic acid. It is disclosed that with this preparation process it is possible to change the interactions between the sol particles, thus causing a large fall in the viscosity of the slurry, which opens up the possibility of working with relatively high solids to liquid ratios.
As already mentioned above, heterogeneous catalysts preferably possess good attrition resistance and high accessibility. In general, as one would expect, the accessibility of the active phase will deteriorate with increasing amounts of binder material. So, the wish for good attrition resistance and high accessibility will usually require a compromise.
There is an ongoing need for catalysts which not only possess good attrition resistance but also a high accessibility of the active phase for the compounds to be converted. A process for the preparation of such catalysts is presented in the present specification.